Merge branch 'kb/preload-index-more'

Use preloadindex in more places, which has a nice speedup on systems
with slow stat calls (and even on Linux).

* kb/preload-index-more:
  update-index/diff-index: use core.preloadindex to improve performance

diff --cc builtin/diff.c
Simple merge

diff --cc builtin/update-index.c
Simple merge